A text subtitle display is a technique for displaying subtitles represented by text codes, in synchronization with each picture of a motion picture. This is one of techniques unique to BD-ROM (Blu-ray Disc Read Only Memory) playback apparatuses. Subtitles can be represented by bitmaps or character codes. The bitmap subtitles are realized in, for example, DVD. The appearances of a bitmap subtitle look to be the same, regardless of playback apparatuses which display thereof. For motion pictures, which put weight on the appearance of images displayed on the screen, there is much point in representing subtitles by bitmaps. On the other hand, in the bitmap subtitle, since each character constituting a subtitle needs to be created, a large size of data needs to be recorded onto a recording medium even when it represents only several characters or several lines of the subtitle.
BD-ROM is a recording medium that has a larger capacity than DVD. BD-ROM has enough capacity to represent subtitles by bitmaps and multiplex them with a video stream. However, when subtitles are distributed via means that is different from BD-ROM (for example, when the subtitles are downloaded via the Internet or supplied to a playback apparatus via a memory card), it is not desirable that the data size of the subtitles is extremely large. Even if the data of the subtitles is managed to be recorded on a BD-ROM, the large data size of the subtitles is expected to cause some problems. For example, if the subtitles are to be resident on a memory, the memory should have a large capacity. Also, it will take a long time to download the subtitles. As a result, it is preferable that the data size is as small as possible.
In contrast, in the text subtitles, the subtitles are represented by character codes. More specifically, the amount of information per character in the subtitles is one byte in ASCII code and two bytes in JIS code. Also, by using a general-purpose character code such as Unicode, it is possible to support a complex character system such as Chinese or Korean, as well as Japanese. When subtitles represented by character code are distributed, only a small data size is required. This is effective when, for example, the data is distributed via the Internet. Under these circumstances, the BD-ROM playback apparatuses, which are expected to be prevalent in future, are planned to be installed with a function to display text subtitles.
When a BD-ROM playback apparatus displays a text subtitle, the BD-ROM playback apparatus performs a control to develop codes constituting the subtitle into bitmaps using the fonts, and display the bitmaps in synchronization with each picture of the video stream.
A conventional technique for displaying text subtitles is disclosed in the following document:
Document 1: Japanese Patent Application Publication No. 2000-228656.